1. Field of the Invention
This invention relates generally to helical scan recording systems and in particular embodiments to an apparatus, system, and method for the self calibration of helical scan read and write heads.
2. Description of Related Art
In helical scan magnetic tape systems, a slow moving tape is wrapped around a cylindrical head drum. The head drum is typically composed of a rotating upper drum that is attached to a stationary lower drum. At least one magnetic read/write head is embedded into the upper drum. The magnetic tape is contiguous with the upper drum and it is positioned at a slight angle to the equatorial plane of the upper drum. A capstan motor is used to transport the tape at a slow speed, relative to the upper drum, and in the opposite direction of the upper drum. Moving the tape in this manner results in a recording format in which successive tracks are written in a helical scan pattern (i.e., diagonally across the tape, from one edge of the tape to the other edge of the tape.) Each track corresponds to one field of data. The angle of the tracks are related to the geometry of the helical scan magnetic tape system, the width of the tape, and the rotation speed of the upper drum.
The lower drum has a precision cut edge that protrudes from the outer surface of the lower drum. The precision cut edge can be used to guide the tape edge and to hold the tape edge in place. The upper drum has an embedded pulse generator (PG) encoder. The PG encoder produces position-related timing pulses. These PG pulses are related to the characteristics of the upper drum (e.g., the rotating speed of the upper drum, the circumference of the upper drum, etc.).
The PG pulse can be used as a reference point for the read and write process. Namely, the PG pulse encoder produces a pulse signal every time the upper drum rotates one revolution. In conventional magnetic tape systems, the position of the read and write heads relative to the PG pulse is often known. That is, when the PG pulse is sensed, the read and write heads tend to be at a known position. The distance between the point at which the PG pulse is sensed and the start of the data tracks may also be known.
This distance value can be used to calculate the time (T.O slashed.) required for the heads to travel from the point at which the pulse is sensed to the start of a data region. Conventional magnetic tape systems may use T.O slashed. to ensure that the read/write heads are properly aligned over the tracks. Specifically, once the PG pulse is generated and sensed, the magnetic tape system waits T.O slashed. seconds, and then begins the writing process. During the read mode (or reading process), the magnetic tape system uses a capstan motor to control the timing, such that the time required for the read head to travel from the point at which the pulse is sensed to the start of a data region is always T.O slashed..
The calculated T.O slashed. value represents the timing of a magnetic tape system when the tape position and alignment are perfectly controlled. Specifically, the tape is maintained at a constant vertical position relative to the cylindrical drum (i.e., the tape does not move up and clown); the read heads are perfectly aligned with the data tracks on the tape before the reading process begins; and this alignment is maintained during the operation of the magnetic tape system.
During the writing process, many factors can affect the timing of magnetic tape system, producing a relative timing that is unequal to T.O slashed.. For instance, dirt build up on the lower drum cut edge or on the capstan motor shaft may cause the vertical position of the tape to vary. When the vertical position of the tape varies, the distance between the point at which the PG pulse is sensed and the start of a data region varies. Therefore, the time required for the read heads to travel from the point at which the pulse is sensed to the start of a data region also varies during the reading process. This varying time could be unequal to T.O slashed.. Thus, using T.O slashed. can cause read errors when the vertical position of a tape varies.
Read errors can also occur when one magnetic tape contains a group (or groups) of tracks written by different magnetic tape systems. Since the T.O slashed. value is generally related to the mechanics of a particular cylindrical drum, each drum may have a different T.O slashed. value due to manufacturing variations. Therefore, the magnetic tape system may be incapable of properly aligning the read heads with each group (or groups) of tracks because the magnetic tape system may only know the T.O slashed. value (and associated distance value) for tracks written by one magnetic tape system.
FIG. 1 shows an exemplary tape 100 that has written data tracks 102. The distances, .DELTA.d.sub.1 104 and .DELTA.d.sub.2 106, represent the distance from the tape edge to start of a data region 108 of the tape 100. Distance .DELTA.d.sub.1 104 is produced by one magnetic tape system and distance .DELTA.d.sub.2 106 is produced by a another magnetic tape system. As observed, .DELTA.d.sub.2 106 is greater than .DELTA.d.sub.1 104. Consequently, the T.O slashed. value for .DELTA.d.sub.2 106 is greater than the T.O slashed. value for .DELTA.d.sub.1 104. The difference between .DELTA.d.sub.2 106 and .DELTA.d.sub.1 104 may be caused by many factors, such as variations in manufacturing, rotating speed of the upper drum, and environmental conditions during the operation of the magnetic tape system.
Minimizing the variation in distances is usually very difficult. Therefore, most conventional magnetic tape systems have calibration systems that recompute the T.O slashed. value when a read error occurs. Some calibration schemes involve control track techniques, automatic track follow (ATF) techniques, and timing tracking techniques.
For the control track technique, a servo write head (embedded in the upper drum) is used to write a control track on the magnetic tape during the write mode. The control track contains a series of 30-hertz pulses. These pulses are used to synchronize the read heads, causing the read heads to pass directly over the previously written data tracks. The control track serves the same general purpose as sprocket holes in a movie film. The sprocket holes help align each frame so that a viewer sees a steady picture on the screen. However, a problem with the control track technique is that it generally requires at least four heads: a data read head for reading data; a servo read head for sensing the control track; a data write head for writing data; and a servo write head for writing the control track.
Using additional servo heads during the read and write process may affect the performance of the magnetic tape system. In particular, before data is written to the tape, the servo write head writes the control track. Hence, the time required for writing data is increased. Similarly, before data is read, the servo read head senses the control track, increasing the time required for reading data. As a result, additional servo heads tend to degrade the performance of the magnetic tape system.
The automatic track follow (ATF) uses four pulses to mark successive data tracks. During the read mode, the read heads sense the ATF pulses. These ATF pulses are usually very low frequency signals and they can be used to provide a position error signal (PES). Based on the PES, the ATF technique continually adjust the read heads during operation of the magnetic tape system, causing the read heads to pass directly over the written data tracks. Unfortunately, the ATF technique lacks accuracy at high track densities. The ATF pulses and data tracks occupy the same data region 112. Hence, the ATF pulses occupy space that could be used by additional data tracks, causing density problems.
In the timing tracking technique, the read heads are locked onto the data tracks using a special synchronization field within the data itself. If the relative timing of the read head, which senses the synchronization field, is known, then a capstan motor (or any tape transport mechanism) can be used to lock the read heads on the data tracks by controlling the timing (i.e., the time at which a read head passes over a portion of the tape). Since the tape edge is mechanically held against the lower drum cut edge, the position of the written data tracks relative to a read head, may vary over the length of the tape. To prevent read errors, the timing tracking technique frequently re-aligns the head with the data tracks.
The timing tracking technique is more accurate than both the control track technique and the ATF technique, but it tends to require frequent calibration during the operation of the magnetic tape system. This frequent calibration slows down the process of transferring data to the magnetic tape.
FIG. 2 represents an exemplary frequent calibration scenario in accordance with the timing tracking technique. The read head senses the start of the data region 108. The read head then performs a first read 202 on a data track 200 that has a distance .DELTA.d.sub.1 104. After an elapsed time, a read error occurs. The magnetic tape system performs a first calibration 206 to establish the correct T.O slashed. for the data track 200 that has a distance .DELTA.d.sub.2. This calibration is performed because .DELTA.d.sub.2 is greater than .DELTA.d.sub.1. The correct T.O slashed. may be stored in the look-up table 204 for subsequent reads. The look-up table 204 contains track numbers and associated T.O slashed. values. Based on the information in the look-up table 204, the magnetic tape system performs a first repositioning 208 of the heads. The second read 210 is then performed. This process of calibration and re-calibration is performed every time a read error occurs. It is conceivable that the tape could continually move up and down during the write process, causing offset written data regions. These offset written data regions are referred to as appends. Appends typically require continual re-calibration. Frequent calibration can produce poor results in audio or video playback because during the calibration, no data is played back. Therefore, the audio and/or video data may be interrupted.
Thus, there is a need in the art for an improved calibration system that maintains alignment between the read heads and the data tracks during the operation of the magnetic tape system, without using additional heads, ATF pulses or frequent re-calibration.